1. Field of the Invention
This invention relates to a focusing screen adapted to be mounted in the optical viewing path of a camera.
2. Description of the Prior Art
Cameras of the single lens reflex type and also of the range finder type generally include a focus indicating system utilizing an image splitting bi-prism. This focus indicating system usually comprises a focusing screen having a focus indicating part disposed in the center of the real image plane of the range finder and comprising image splitting prisms.
The focus indicating system of such a split-image type, as compared with other systems, has a good focusing accuracy, but suffers from a phenomenon that the focus indicating part dark-changes for an interchangeable lens having a great F-number, namely, low in brightness.
As the aperture ratio of the lens is decreased, the image splitting bi-prism has increasingly larger dark areas. Accordingly, the focusing becomes more difficult with the increasing of the F-number of the lens.
This focus indicating accuracy and the dark of the focus indicating part have a relation contrary to each other (inversely proportional to each other) for the vertical angle of the image splitting prism. This contrary relation is attributable to the fact that, among the light beams from the exit pupil of the lens, only the light beam having a particular angle of incidence determined by the vertical angle of the image splitting prism passes through the range finder and reaches the eye of the observer and contributes to the focus indicating. That is, if the vertical angle of the splitting prism is increased, the light rays from the marginal portion of the exit pupil of the phototaking lens can be directed to the eye-piece of the view finder and thus, the focus indicating accuracy is enhanced. However, a splitting prism having a great vertical angle has its focus indicating part dark-changeable even for an interchangeable lens which is a little dark, namely, which has a great F-number, and thus, the focus indicating becomes impossible.
Conversely, if the vertical angle of the prism is made small, the focus indicating accuracy is reduced but the focus indicating is possible even for an interchangeable lens which has a great F-number, namely, which is considerably dark.
To overcome such phenomenon, the vertical angle of the image splitting prism usually is not rendered to so great a value but is minimized to an angle in the vicinity of 8.degree. so that, even if the focus indicating accuracy is sacrificed to some extent, the focus indicating becomes possible even for a dark interchangeable lens having an F-number of the order of 4.5-5.6.
However, for interchangeable lenses of a smaller maximum aperture ratio, for example f/8 and larger, the aforementioned problem of dark prism is again presented.
U.S. Pat. No. 2,881,686 shows a focusing screen having a circular Fresnel lens formed on its upper surface. However, this Fresnel lens does not contribute to the focus indicating function.
British Pat. No. 961,033 and French Pat. No. 1,157,316 show a focusing screen having a Fresnel-like split-image bi-prism. However, there is no disclosure in these references of the advantage of facilitating the range finding operation by the use of image blurring and also of alleviating darkening.
U.S. Pat. No. 4,003,637 discloses a focusing screen comprised of microprisms, but again in this reference, there is no disclosure of a technique for solving the above-noted problem.
A focusing screen overcoming the above-described technical disadvantages has already been proposed in U.S. Application Ser. No. 204,009, now U.S. Pat. No. 4,338,012, which issued July 6, 1982, filed by the assignee of the present invention. The present invention is an improvement over such a focusing screen.